Congestive heart failure is a major cause of death and disability in the United States. While some cardiac patients respond to pharmacological therapies, the alternative treatment for a patient who does not respond to conventional therapies is a heart transplantation. Heart transplantations generally require the patient to wait for a donated heart; so as a bridge to a heart transplant, many patients may rely upon mechanical assistance provided by a left ventricular assist device (“LVAD”) for temporary assistance, as well as a destination therapy.
A typical prior art LVAD, as shown in FIG. 1, consists of the blood pump 1, an inflatable bladder; an internal conduit 2 connecting the blood pump to the percutaneous access device (“PAD”); the PAD 3, a through-the-skin port that permits power, electrical signals and internal fluid (typically air) to pass between the drive unit and blood pump; and the external drive unit 5, a device powering and controlling the blood pump. The inflatable bladder typically either rings the aorta or is sutured thereto. The PAD 3 allows the implanted blood pump 1 to be operatively connected to or disconnected from the external drive unit 5. To inflate the blood pump 1, pressurized air is supplied from the drive unit compressor (not shown). The air flows from the compressor via an interconnect line through a valve manifold in drive unit 5 to an external drive line 4 running to the PAD 3 and then through the implanted internal drive line 2 to the blood pump 1. Alternatively, an isolation chamber, separating the pressure (or vacuum) source from the air flowing to the pump, can be used to isolate the subcutaneous portion of the pneumatic circuit from the supercutaneous portion.
An integral component of an LVAD system is the controlled inflation of the pneumatic blood pump component of the LVAD. In the CardioVad system made by L. VAD Technology, Inc., there are two types of drive units that control inflation of the blood pump. The first type is the line-powered drive unit (“LDU”). The LDU is run by household current and the power cord severely restricts the patient's mobility. The second type is the wearable drive unit (“WDU”). The WDU, as suggested by its name, is a battery-powered unit worn within a specially designed vest or belt, and allows the patient the mobility not permitted by the LDU.
When WDU trials began with human patients, some patients complained that they felt a sharp, abrupt pain or “kick” in their chest when the blood pump was operating. The origin of the sharp pain was traced to the rapid inflation of the blood pump caused by the initial release of air from the compression tank at the beginning of a pump inflation cycle. Although the precise physiological cause of the pain has not been determined, it is generally attributed to the strain on the aorta and surrounding, interrelated biological features, caused by the inflation of the pump.
The pneumatic design of the WDU allowed the initial release of air to enter the pump at a high pressure, with a peak value of approximately 200 mm Hg. Because of differences between the LDU's closed loop pneumatic design and the WDU's open loop design, the LDU operates with a peak pressure of approximately 175 mm Hg and does not have the “kick” problem of the WDU. Thus, based on clinical experience with these LVADs, a system is desired that can reduce patient pain associated with the “kick,” while still meeting all the important operating specifications.
U.S. Pat. No. 5,904,666 to DeDecker and Freed (entitled “Method and Apparatus for Measuring Flow Rate and Controlling Delivered Volume of Fluid Through a Valve Aperture”) discloses a technique for estimating the volume of fluid flowing through a valve in a pneumatic LVAD, using a valve having a fixed aperture across which pressure measurements are made. The differential pressure values are used to determine the total volume of fluid passing through the valve. This patent discloses a system for measurement of a volume of air delivered to inflate the blood pump, but discloses nothing to address the problem of regulating the peak pressure of the air exiting the valve to the blood pump to prevent the “kick” problem under all operating conditions.
U.S. Pat. No. 6,042,532 to Freed and Psakhis (entitled “Pressure Control System for Cardiac Assist Device”) discloses a control scheme for changing the pressure in the air tank in a pneumatic LVAD so that the blood pump is fully inflated in a desired length of time. This patent discloses the same valve system and flow measurement scheme as disclosed in U.S. Pat. No. 5,904,666, but discloses nothing to address the potential “kick” problems inherent in that system.
U.S. Pat. No. 6,735,532 to Freed, Psakhis and DeDecker (entitled “Cardiovascular Support Control System”) discloses control techniques for measuring pneumatic LVAD patient parameters, along with methods for timing the inflation and deflation of the blood pump. This patent discloses the same valve system and flow measurement scheme as disclosed in U.S. Pat. Nos. 5,904,666 and 6,042,532, but discloses nothing to address the potential “kick” problems inherent in that system.
The prior art designs of the drive unit were developed to address problems of optimizing the timing and volume of air delivered for inflation of the blood pump within a wide range of heart rates (e.g. 30 to 180 beats per minute) and responding to fluctuations in the patient's heart rate. However, these known designs do not control the peak pressure of the initial release of air to the blood pump, other than by establishing a maximum value based on the need to avoid rupturing the bladder. The valve controls of the prior art drive units modulate the timing and volume of air for the inflation and deflation cycles, and automatically adjust the timing to compensate for changes in heart rate. None of the prior art discloses any technique for controlling the peak pressure of the initial release of air to the blood pump, to prevent the “kick” problem seen in some drive units. Thus, there is a need in the art for an LVAD pump inflation system that can reduce or eliminate patient discomfort from the inflation “kick” while still meeting all important operating specifications.